In the past I've always done my own moving. I'm also curious on how others will answer this. In the meantime, I found these tips on tipping:

-Moving is hard, back-breaking work so it would only be common decency to have cold bottled water, soda or sport drinks on hand.
-Some people prefer to buy the crew lunch in lieu of a tip and this is perfectly fine. Some do both. It's whatever you are comfortable with. If you do offer to buy the crew lunch, make sure to ask if they have a preference as to food choice. Many people think, "Oh, we'll get some pizza," so you can imagine how much pizza these guys might have had that month.
-You will hear varying advice on how much to tip. Percentages don't really work as they do in restaurant work. The difference between a $2,000 full-truck move and a $20,000 full-truck move is likely the distance the moving van travels, not how much time it took to load the vehicle. Basically, if it's a half-day (4 hours) move, $10 per person is considered appropriate. If it's a full-day move (8 hours), then $20 is the consensus. If you have a lot of heavy furniture, narrow or winding stairs, a steep lot, etc., you should consider adding to the amount. If the crew works 12 hours to get your belongings packed, figure $40 as fair.
-A tip about tips from movers themselves: Donít give the lump some to the foreman or driver. Give each worker their tip. First, it shows that you recognize and appreciate their individual efforts. Second, there are some unscrupulous foremen, who will keep the entire amount for themselves.
-Avoid giving the guys beer at the end of the day. It's a bit stereotypical and most legitimate moving companies have rules against drinking on the job. It also opens liability issues. Make the tip cash and the movers can spend it how they wish.

We didn't tip the movers anything. Maybe it's the fact that we aren't Americans so tipping isn't our cultural norm. We were paying thousands of dollars for the service as it was, it's hardly like restaurants where the value of their product is a few dollars. Our mentality is if you're paying that much for the service, you shouldn't have to tip on top of it. I do remember offering to buy them some lunch but I can't remember if it was accepted or not.